IGRT is a widely used medical technique. For instance, an IGRT apparatus may include a CT imaging device and an RT device that share a same couch for supporting an object during an examination. However, compared to the couch of a conventional CT, the radiotherapy couch needs to extend longer from its support position into the bore of the CT gantry. As a result, the couch may sag or deflect along the longitudinal direction. The deflection of the couch from a horizontal baseline may be a function of both the weight of the object and the amount by which the couch extends along the longitudinal direction of the couch. Thus, it may be desirable to develop a method or a system to compensate the sag of a couch that may remove or reduce the error of object position to improve the quality of a radio therapy.